Post-transcriptional gene silencing by RNA interference in non-mammalian vertebrate systems: where do we stand?

RNA interference (RNAi), the process by which double stranded RNA induces the silencing of endogenous genes through the degradation of its correspondent messenger RNA, has been used for post-transcriptional gene silencing allowing scientists to better understand gene function, becoming a powerful tool in reverse genetics for in vivo and in vitro systems. Successful results in vivo have been obtained from invertebrate animal models, whereas vertebrate systems have been limited primarily to mammalian models and cell lines. Nevertheless, exciting results have also been reported from non-mammalian vertebrate models, such as the knock-down of endogenous genes in Xenopus tadpoles by a construct containing both a Xenopus-specific shRNA sequence and the human Ago2 (which is a key enzyme in the RNAi silencing complex), or the design of a novel vector expressing a miRNA driven by a tissue-specific promoter in zebrafish, and the use of an avian retroviral vector to deliver miRNA and shRNA in chicken embryos proving to be effective in knocking-down endogenous genes with a long lasting effect, to mention some examples. Whether dsRNA is able to initiate a specific RNAi response, or all the factors required for RNAi are present in non-mammalian vertebrates, are still questions which remain to be answered. Further progress in understanding natural RNAi mechanisms in non-mammalian vertebrates will help scientists to overcome difficulties and improve this gene silencing technology. There is no doubt that in few years RNAi silencing approaches will become the tool of choice to knock-down genes in all groups of non-mammalian vertebrates, fulfilling different purposes, from basic research to animal therapeutics and drug discovery.